On Thu, 23 Nov 2023 02:07:59 GMT, David Holmes <dhol...@openjdk.org> wrote:
>> In the rewrites made for: >> [JDK-8318757](https://bugs.openjdk.org/browse/JDK-8318757) `VM_ThreadDump >> asserts in interleaved ObjectMonitor::deflate_monitor calls` >> >> I removed the filtering of *owned ObjectMonitors with dead objects*. The >> reasoning was that you should never have an owned ObjectMonitor with a dead >> object. I added an assert to check this assumption. It turns out that the >> assumption was wrong *if* you use JNI to call MonitorEnter and then remove >> all references to the locked object. >> >> The provided tests provoke this assert form: >> * the JNI thread detach code >> * thread dumping with locked monitors, and >> * the JVMTI GetOwnedMonitorInfo API. >> >> While investigating this we've found that the thread detach code becomes >> more correct when this filter was removed. Previously, the locked monitors >> never got unlocked because the ObjectMonitor iterator never exposed these >> monitors to the JNI detach code that unlocks the thread's monitors. That bug >> caused an ObjectMonitor leak. So, for this case I'm leaving these >> ObjectMonitors unfiltered so that we don't reintroduce the leak. >> >> The thread dumping case doesn't tolerate ObjectMonitor with dead objects, so >> I'm filtering those in the closure that collects ObjectMonitor. Side note: >> We have discussions about ways to completely rewrite this by letting each >> thread have thread-local information about JNI held locks. If we have this >> we could probably throw away the entire ObjectMonitorDump hashtable, and its >> walk of the `_in_use_list.`. >> >> For GetOwnedMonitorInfo it is unclear if we should expose these weird >> ObjectMonitor. If we do, then the users can detect that a thread holds a >> lock with a dead object, and the code will return NULL as one of the "owned >> monitors" returned. I don't think that's a good idea, so I'm filtering out >> these ObjectMonitor for those calls. >> >> Test: the written tests with and without the fix. Tier1-Tier3, so far. > > test/hotspot/jtreg/serviceability/jvmti/GetOwnedMonitorInfo/GetOwnedMonitorInfoTest.java > line 59: > >> 57: // GetOwnedMonitorInfo testing. >> 58: Object obj = new Object() { public String toString() {return >> "";} }; >> 59: jniMonitorEnter(obj); > > I would add a check for `Thread.holdsLock(obj);` after this just to be sure > it worked. Done > test/hotspot/jtreg/serviceability/jvmti/GetOwnedMonitorInfo/GetOwnedMonitorInfoTest.java > line 61: > >> 59: jniMonitorEnter(obj); >> 60: obj = null; >> 61: System.gc(); > > Again one gc() is generally not sufficient. > > How can this test tell that the object in the monitor was actually cleared? I > think `monitorinflation` logging may be the only way to tell. Yes, probably. I've been looking at the `monitorinflation` logging to very that it gets cleared. I think it would be messy to try to get this test to also start to parse logs. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/16783#discussion_r1403245976 PR Review Comment: https://git.openjdk.org/jdk/pull/16783#discussion_r1403244666
